[Doc] Remove absolute internal links from doc

[balat]

[balat]

• [ ] Prefer relative links to absolute. Remove all [[https://ocsigen.org/ ... ]], and avoid [[/ ... ]]
• [ ] Prefer links of the form <<a_manual>> or <<a_api>>`` to relative links to site pages (as manual or api pages could move)
• [ ] Use [[wiki("toto"):]] for links to other projects if a_manual or a_api is not possible
• [ ] Replace links to ocsigen1.inria.fr by links to ocsigen.org (as inria may not host our servers forever)